I am using a small base circle cam on my new 406 and I have to use .150 oversized pushrods because the lifters are farther down it the lifter bore. My problem is that since I am using longer pushrods it pushes the rocker arm up higher on the screw in stud. It's so far up the stud that I can only get about 2 threads onto the rocker arm nut. It seems to me that I need to get a stud that is at least .375 longer so that there will be enough threads to catch and hold. ARP has a stud that is only .145 longer. My springs have quite a bit of spring pressure so I want as many threads as possible. Oh, I used a pushrod length checker to determine the length that I needed.

Any of you ever ran into this problem?

 

How did you determine the correct push rod length?

Typically, the valve train geometry is correct when the rocker arm is perpendicular to the stud at about 1/3 total lift of the cam lobe:

 

Getting the longer push rod was to counter the effect of the lifter being lower due to the base circle being lower. So the height of the pushrod coming up through the head should be about the same as it was with a regular cam. Is it a roller cam? Did you perhaps get pushrods that are .150 longer than stock flat tappet size push rods and are using them on roller cam lifters?

 

I figured the length by using a pushrod length checker. It is the kind that sits on the rocker stud like a rocker arm. I talked to Comp Cams today and determined that I needed shorter pushrods instead of longer ones. I goofed up but now I've got it figured out. Thanks guys.

 

A lot of that will depend on the heads being used. 18° heads with longer valves (like Ford 351W valves) and shafted rockers will all change the geometry. That's why the "1/3" lift rule" usually works the best - at least for me. Of course, that's only one aspect of correct valve train geometry. There are still the issues of cleance around the springs and retainers, and contact patch on the valve tip. With stock heads, valves, and rockers, that's usually not a problem, and the plastic push rod length checkers can be O.K.

 

Vader, how do I go about checking at 1/3 lift? Is it 1/3 of the cam lobe lift or 1/3 of valve lift? I guess I can take the lift and divide by 3. The number that I come up with is what I should let one side of the rocker come up. Then check to see if it is perpindicular to the stud. Can I just tell by eyeballing it?